Overhead cranes which travel on their wheels along spaced apart generally parallel rails are subject to the continuous problem of the skewing of the crane on the rails. The forces causing skewing are due to rail displacement caused by rail support changes, rail deterioration resulting from improper adjustment of acceleration and deceleration forces of drive motors and brakes, and variations in traction due to rail contamination from moisture vapor and airborne particles. The skewing itself exacerbates the problem since it produces stresses on the rail structure which contribute further to the displacement of the rails. Moreover, the skewing causes severe stressing and wear of the crane wheels. The end result of rail displacement and deterioration and consequent increased skewing is a short wear life of the rails requiring their relatively frequent replacement and very frequent replacement of the wheels.
Various prior art solutions to the skewing problem have been developed. These include controls in which a sensing device is used for detecting skew and adjusting the drive motors of the crane to correct the skew. For example, in a crane having driving wheels at opposite bridge ends of the crane independently driven, slowing the motor of the drive wheel at the leading skewed bridge end will correct the skew. Another approach, upon sensing skew of the bridge, is to either apply a friction drag to the leading skewed end of the bridge or activate a wheel brake on the leading drive wheel of the skewed bridge. A further solution, disclosed in U.S. Pat. No. 3,095,829 to Dehn, in a crane having drive wheels driven and controlled independently, is to decrease the clearance between the rail and the outside flange of each of the drive wheels. Consequently, the outside flange of the leading drive wheel, when the crane moves to a skewed position, will contact the outer side of the rail on which it rides and cause that wheel as well as its drive system to slow down due to the resulting friction and thereby correct the skew. The skew sensing devices used in prior art skew correction methods have typically been contacting devices such as rollers which are connected to switches and proximity type switches mounted on the crane which will provide an output signal indicative of their distance from the rail.
The problem with the prior art anti-skewing devices is that they rely on either a separate drive for the drive wheels on the opposite ends of the crane bridge or on variable speed drives so that one wheel can travel at a different speed than the other. With these types of drive systems, it is possible to slow the lead wheel in a suitable manner so that the crane returns to a parallel running position relative to the rails on which it travels.